14 Kidneys and Renal Transplants

The interlobar arteries supply the medullary pyramids and run between the pyramids to the corticomedullary junction. There they branch dichotomously into the arcuate arteries, which supply the glomeruli via the interlobular arteries and afferent vessels. Individual interlobular arteries are distributed to the renal surface where they anastomose with the capsular arteries (superior, middle, and inferior capsular arteries). The inferior phrenic arteries, lumbar arteries, suprarenal arteries, and testicular artery may function as collateral pathways, usually in response to renal vascular occlusion.³⁹

14.2.2 Variants

Due to the complex embryonic development of the kidneys, the examiner may be faced with numerous variants in renal position and vessel origins. Anatomic variants such as supernumerary kidneys, ectopic kidneys, malrotation, and horseshoe kidneys are associated with atypical vascular patterns. The kidneys have duplicated arteries in 24% to 25% of cases, and three or more arteries are present in up to 3% of cases. Confusion may also result from aberrant polar vessels and arteries arising from iliac or visceral branches of the aorta, which are often detectable only by angiography.

14.3 Examination Technique

▶ Transducer and settings. The recommended transducer for renal imaging in adults is a curved-array or phased-array transducer with an operating frequency of up to 7 MHz. A transducer in the 5- to 15-MHz range may be preferable for thin patients and children. The power output and time-gain compensation for renal flank scans are initially adjusted to produce a homogeneous echo texture in the renal parenchyma. The upper renal pole is more posterior than the lower pole, and the transducer orientation should be aligned to the renal axis. Next the aorta is identified, and the time-gain compensation, focus, and gain are adjusted so that the aortic lumen and proximal renal arteries are essentially echo-free. Once the B-mode image has been optimized, color Doppler is switched on.

▶ Preparations. Before a Doppler ultrasound examination of the kidneys, it should be confirmed that the kidneys are in an orthotopic position and any rotational variants should be noted. Renal size, parenchymal thickness, and the echogenicity of the cortex and pyramids are evaluated. The area should be checked for perirenal fluid collections, not only after trauma but also in patients with nephritis, acute pancreatitis, or lymphoma as the presence a fluid collection may have major diagnostic implications. Moving the patient to a slight left or right lateral decubitus position may facilitate renal scanning. The degree of bladder distention can affect the width of the renal pelvis and ureter.

14.3.1 Procedure

▶ Extrarenal arteries. The origins of the renal arteries can often be visualized while the patient is still in a supine position. Viewed in the frontal plane, the right renal artery arises from the aorta at approximately 11 o’clock and the left renal artery at 5 o’clock. Next the right renal artery is visualized posterior to the vena cava in a longitudinal scan. Generally, it can be identified as a pulsating dot slightly indenting the posterior wall of the vena cava. The vessel should be checked for possible duplication or multiplicity. To define the course of the renal arteries on the right side, the patient should be turned 30 to 45 degrees to the left side. First the renal hilum, inferior vena cava, and aorta are identified in transverse scans. In most cases the full length of the renal artery can be displayed at full inspiration. For the left renal artery, the same procedure is followed as in right lateral decubitus.

A coronal scan from the right flank in thin patients can display the V-shaped origins of both renal arteries (“banana peel sign”). This longitudinal view is also recommended for localizing the renal artery origins in patients with an abdominal aortic aneurysm.

CEUS is not widely utilized for the detection or exclusion of renal artery stenosis, and Doppler techniques continue to be the mainstay for this indication. Contrast arrival in the renal parenchyma is delayed in patients with intrarenal vascular disease, severe renal artery stenosis, or—due to low output—in cardiac failure. While the arrival time in healthy patients is approximately 8 to 12 s, cardiopulmonary diseases delay contrast arrival in the renal artery, whereas in young subjects the arrival time tends to be shorter (about 8 s). After very rapid enhancement of the cortex, enhancement of the pyramids spreads from the outer to the inner medulla (Fig. 14.1).

CEUS is most commonly used for the investigation of focal renal lesions or suspected renal infarction. As explained in Chapter 4, the contrast dose depends on the type of scanner used, the transmission frequency, and the CEUS technique, i.e., the pulse inversion technique or amplitude modulation technique. In most cases a somewhat lower dose can be used than for liver imaging, provided there is a clear acoustic window to the kidneys.

The contrast agent has a very short transit time of approximately 2 s, which corresponds to high, physiologic renal blood flow. The effect of other enhancing organs like the liver and spleen should be noted in ultrasound contrast examinations of the kidney since high uptake at those sites leads to sound attenuation at the renal level as soon as the sound beam passes these organs before entering the renal parenchyma.

¹Lumason is known in Asia and Europe as SonoVue.

14.4 Normal Findings

Due to the low intrarenal resistance, renal blood flow is constant and directed toward the kidney; this differs from the patterns seen in the distal aorta and pelvic/lower extremity arteries. On the one hand, increased Doppler frequency shifts due to the insonation angle should not be mistaken for stenosis. But at the same time, any red-to-blue color reversal that is noted with proper equipment settings should be taken as evidence of disease. Fig. 14.2 illustrates the appearance of the renal arteries in different imaging modes.

▶ Variants. A small-caliber vessel (< 4 mm) supplying a normal-sized kidney should raise suspicion of multiple renal arteries.⁴ The right renal artery may run anterior to the vena cava as a normal variant (Fig. 14.3).

In another normal variant, the left renal vein may run posterior to the aorta (Fig. 14.4) sometimes accompanied by a second, smaller vein passing anterior to the aorta (circumaortic left renal vein).

An apparent parenchymal defect, usually located in the lower third of the kidney, may represent an accessory hilum. A lower pole vessel may be visible at that site and can be traced to its origin from the aorta or iliac artery (Fig. 14.5).

14.4.2 Intrarenal Arteries

In slim patients the intrarenal arteries can be defined to the level of the arcuate arteries and interlobular arteries. The interlobular arteries form a palisade-like arrangement of closely packed vessels. In color duplex sonography, the colors often blend together due to the “blooming artifact.” Increasing the PRF in this situation will then cause loss of sensitivity for defining small vascular branches. Under favorable scanning conditions, however, this problem does not occur in B-flow imaging.

Following contrast administration, enhancement of the aorta is followed immediately by contrast arrival in the renal vessels. The time intensity curve (TIC) shows a rapid rise of intensity (Fig. 14.1). Individual parenchymal vessels may be visualized in the early arterial phase, depending on the enhancement technique. Flow signals uniformly permeate the parenchyma in power Doppler mode or after contrast administration. Fig. 14.6 shows the appearance of parenchymal vascularity in power Doppler, B-flow, and after contrast administration.

The Doppler spectra sampled from intrarenal arteries normally show a narrow systolic peak with continuous antegrade flow in diastole. The peak systolic flow velocity (v_maxsys) and peak end-diastolic flow velocity (v_maxdia) are determined in the segmental arteries at the junction of the renal pelvis and parenchyma.

The reference range for the resistance index (RI) is from 0.52 to 0.7 (Fig. 14.7). The RI value is age-dependent and is high in early childhood, at approximately 0.70. It falls rapidly to values of approximately 0.55 by about 10 years of age, then rises again with age.⁴⁰

Thus, the normal mean RI in healthy children under the age of 6 years, at > 0.70, is higher than in adults.⁴⁹ This is most likely due to the ongoing maturation of the kidneys and a higher plasma renin activity than in adults. RI values between 0.62 and 0.75 are still considered normal in older adults.⁴⁰

The RI provides indirect information on vascular resistance in arteries distal to the sampling site; thus, it is a nonspecific value that may be altered by many pre-, intra-, and postrenal diseases. It is calculated from the peak systolic velocity (v_maxsys) and peak end-diastolic velocity (v_maxdia) and is therefore angle-independent:

RI = 1–(v_maxdia/v_maxsys)

RI is an important but nonspecific parameter for assessing the severity and prognosis of vascular and interstitial diseases of the kidney (Table 14.1).^{18 ,​ 52 ,​ 110} Low and elevated RI numbers can also be expected in patients with aortic valve stenosis or aortic valve insufficiency. Besides the familiar B-mode criteria and RI values, elastography may also contribute to the assessment of primary and secondary renal disorders.^{16 ,​ 96} Recent studies have shown that the peak systolic velocity correlates better with the severity of chronic kidney disease. The RI value is a reliable parameter for detecting progression to end-stage disease and is also relevant for the use of nephroprotective therapy.¹⁶

Table 14.1 Frequent causes of an increased or decreased RI value

14.5 Pathologic Findings

The most common renal pathologies are stones, pyelonephritis, and primary and secondary renal tumors.

Potentially obstructive stones (calculi) in the kidney, ureter, and bladder may produce a twinkling artifact due to their rough surface. Because this is an artifact, a higher PRF is used for color Doppler scans and the color box is positioned over the questionable site.²⁵ Calculi are associated with a typical color-reversal artifact: a comet tail of varying color, either V-shaped or with concave sides, frequently appears behind the stone (Fig. 14.8).

Escherichia coli is the most common organism that can ascend from the bladder into the ureter and renal pelvis. Its toxins can reduce the muscular tone of the ureter, which further facilitates the ascending infection. Because the route of infection continues across the calyces to the renal parenchyma, a lobular inflammatory parenchymal reaction may occur; this may involve single or multiple lobules or all of the parenchyma. This process incites an inflammatory vasoconstriction and consequent ischemia, which may progress further to form a regional abscess. The surrounding area displays inflammatory edema (pseudotumor, Fig. 14.12) and hyperemia of the pararenal fat.

Thickening of the ureteral wall and pelvis are important B-mode signs of inflammatory edema. Renal sweating is an important sign of local perirenal edema (Fig. 14.9, Fig. 14.10); it consists of a small fluid collection, usually of a bizarre shape, found on the renal capsule. If scanning conditions permit, a high-frequency linear-array transducer should be used to look for perirenal edema. This finding is not specific for inflammation, as it also occurs in acute renal failure. Similar changes occasionally result from infiltration by non-Hodgkin lymphoma, in which case the clinical presentation and ultrasound findings would be indicative of systemic disease (see Fig. 14.11).

Lobular pyelonephritis develops as a result of local edema, appearing as enlarged parenchymal areas that can mimic a tumor when the disease is at an early stage (Fig. 14.10, Fig. 14.12). The affected areas appear hypovascular on color duplex sonography. Abscesses usually appear as hypoechoic to echo-free masses in the renal parenchyma that have an avascular center (Fig. 14.13).

14.5.3 Tumors and Tumor Vascularity

▶ Tumor detection. The sensitivity of an imaging procedure for detecting renal tumors, metastases, and renal infiltration by lymphoma depends on the size, location, and acoustic properties of the tumor. In the case of small renal cell carcinoma (RCC), a standard nonenhanced ultrasound examination is inferior to contrast-enhanced CT.³⁸ This is mainly because small tumors are often isoechoic to surrounding renal parenchyma, and renal tumors are difficult to detect sonographically if they do not transcend the organ boundaries.¹⁵

Color duplex sonography does not have a role in tumor detection. While CT and MRI are performed almost exclusively with contrast medium, CEUS has essentially no role in the primary detection of renal carcinoma. Small renal masses of less than 2 to 3 cm in diameter were formerly classified as renal adenoma or small, nonmetastasizing, slow-growing renal carcinoma.⁸ Primary or secondary renal masses not visible on B-mode images can usually be detected and localized with CEUS, just as in CT or MRI.

▶ Tumor characterization. Color duplex sonography is not a mainstay for lesion characterization, i.e., the differential diagnosis of different tumor entities. Color duplex sonography can be very helpful, however, for detecting or excluding tumor invasion of the renal venous system—although CEUS has better sensitivity and can positively distinguish between tumor invasion and coagulation thrombus.

Because a very high percentage of solid renal masses are malignant (approximately 80%), an imaging procedure must have very high specificity for the confident differentiation of benign and malignant renal masses. Tuncali et al reported on 27 patients with small, solid renal masses of up to 2 cm in size, which were classified as RCC by CT.¹⁰⁵ When the lesions were biopsied prior to planned radiofrequency ablation, 10 of the 27 patients were found to have a benign mass.

Imaging is of limited value in differentiating the most common benign lesions such as angiomyolipoma, oncocytoma, and hemorrhagic cysts from malignancy. Oncocytomas often have a good blood supply and show little washout. Even B-mode imaging can provide presumptive evidence of intratumoral hemorrhage. Intracystic hemorrhage should be considered suspicious until an underlying tumor has been positively excluded and should be investigated by CEUS (Table 14.2)). Primary RCC requires differentiation from renal metastases and infiltrates associated with systemic diseases.

Table 14.2 Criteria for the enhancement characteristics of solid renal tumors in CEUS

Renal Cysts

▶ Incidence. Renal cysts are by far the most common focal renal lesions. They are found in about two-thirds of all patients over 80 years of age, with a slight male preponderance.

▶ Classification. Cysts are classified by their location (perirenal, parenchymal, or peripelvic) or their malignant potential. Simple cysts can be characterized sonographically with reasonable confidence. Complicated cysts are those with a thickened wall, foci of calcification or sclerosis, septa, internal echoes, and solid-appearing components.

The Bosniak classification is widely used for the benign/malignant differentiation of complicated cysts in contrast-enhanced CT. This classification is based on the enhancement characteristics of the cyst wall and contents. Simple, uncomplicated cysts in category I do not enhance after contrast administration. Small areas of wall calcification or sclerosis or the presence of thin septa are assigned to category II if they are nonenhancing. Cysts with multiple or thickened septa are placed in category IIF and should be followed (“F” stands for “follow-up” as 1% to 5% of IIF cysts may turn out to be malignant). Categories I to IIF do not require a surgical diagnosis.

Cysts with an enhancing wall, thickened septa, or thickened components are classified as category III and are an indication for surgery. This category could technically include infected cysts with a hyperemic wall, so clinical parameters should determine the appropriate mode of treatment. In a large review study, the likelihood of malignant lesion in category III cysts is estimated as only 51%, so nearly half of the surgically treated patients were overtreated.¹¹⁸ The presence of a larger, solid, enhancing component occupying < 50% of the cyst signals a category IV lesion, which has almost a 100% likelihood of malignancy. The hallmark for category IV lesion is an RCC with cystic or necrotic changes. The CT classification is applied to CEUS in Table 14.3.

Table 14.3 Classification of renal cysts by CEUS, based on the Bosniak classification

▶ Findings. The detection of tumor tissue relies on the enhancement of tumor vessels. If this feature is absent as in Bosniak categories I to IIF, no viable tissue is present (Fig. 14.14, Fig. 14.15). During follow-up of category IIF cysts, an increase of complexity or growth of theses cystic lesions is mostly not caused by a malignant transformation.^{2 ,​ 9 ,​ 26 ,​ 37 ,​ 92} Compared with contrast-enhanced CT, the various cyst categories are often assigned to a different grade by CEUS owing to its higher temporal, spatial, and contrast resolution.

Today CEUS plays a major role in the characterization of complicated cysts or inconclusive CT/MRI findings, and many experts consider it the gold standard. But even CEUS has limitations relating to the type of equipment used, the contrast dose, and, in the case of a calcified cystic wall or large cysts with small solid components, incorrect placement of the region of interest (ROI) during the brief wash-in phase.

▶ Differential diagnosis. An important differential diagnosis is benign cystic nephroma, a very rare epithelial renal stromal tumor (Fig. 14.16) that is more common in males in childhood cases and more common in women in adult cases. While the adult form of cystic nephroma is believed to be part of mixed epithelial stromal tumor (MEST), cystic nephroma in childhood is associated with mutations in the DICER1 gene.¹¹⁹ This tumor shows a somewhat gradual spread of enhancement on CEUS and is always diagnosed histologically.^{64 ,​ 90}

Angiomyolipoma

▶ Incidence. Angiomyolipomas comprise the largest percentage of benign solid renal tumors. Autopsy studies show an overall prevalence of approximately 0.3%. Females are predominantly affected by a 4:1 ratio.³² About 80% of patients with tuberous sclerosis (Bourneville-Pringle disease) have angiomyolipomas in the kidney.

▶ Classification. The congenital form of angiomyolipoma, which occurs in the setting of tuberous sclerosis and is characterized mainly by cerebral and typical cutaneous changes, requires differentiation from sporadic angiomyolipomas, which may occur as single or multiple lesions.

▶ Risks. Angiomyolipomas do not undergo malignant transformation; their true risk is intralesional hemorrhage, which may become life-threatening. The likelihood of hemorrhage depends on tumor size and is absent in angiomyolipomas of less than 4 cm in diameter. The rupture of intralesional aneurysms is responsible for the hemorrhage.¹¹⁶ The lesions are detectable by color duplex sonography, but CEUS is more sensitive.

▶ Findings and differential diagnosis. The sonographic features of angiomyolipomas depend on the relative proportions of tissue types comprising the tumor: vessels, smooth muscle, and fat. In the great majority of cases the fatty component is predominant and accounts for the echogenic appearance of the tumors, which are usually small (< 2 cm). Typical angiomyolipomas have a sharply circumscribed, snowball-like appearance and may transgress the renal boundary (Fig. 14.17, Fig. 14.18, Fig. 14.19).

Intratumoral calcifications do not support a diagnosis of angiomyolipoma and are more characteristic of RCC. A slight acoustic shadow like that seen at higher insonation frequencies and in tissue harmonic imaging (THI) is more consistent with angiomyolipoma than RCC but is not a definite differentiating criterion.⁸⁸ Because RCCs smaller than 3 cm may be more echogenic than the renal parenchyma in approximately 30% of cases, the differentiation between echogenic angiomyolipoma and RCC is sometimes uncertain.²²

Color duplex sonography often does not demonstrate tumor-feeding vessels in small angiomyolipomas. Intratumoral vessels are most commonly found in larger angiomyolipomas (Fig. 14.17, Fig. 14.19).

There is disagreement about whether CEUS may be helpful in differentiating between RCC and angiomyolipoma. Recent studies have found prolonged, centripetal enhancement to be an important differentiating feature of angiomyolipoma from RCC.⁵⁵ But a very similar enhancement pattern can also be found in RCC, which limits its value as a differentiating criterion (Fig. 14.18, Fig. 14.24). Oncocytoma (p. 438) may also exhibit rim enhancement (Fig. 14.20, Fig. 14.21). Consequently, there is no reliable criterion in CEUS for diagnosing angiomyolipoma.

Unenhanced CT or MRI can usually establish the diagnosis of angiomyolipoma by their ability to detect fat on native,¹² nonenhanced scans, although 5% of angiomyolipomas, usually of smaller size, do not contain detectable fat. In case of a doubtful fat-containing renal mass, biopsy may be a diagnostic option. Thus, CT and MRI continue to be the mainstays in differentiating these tumors from RCC.⁹

The value of color duplex sonography and CEUS lies mainly in the assessment of potential hemorrhage risk. Intratumoral vessels are detectable by color duplex sonography, but this can be done more reliably with CEUS (Fig. 14.19). Aneurysm detection constitutes an urgent indication for surgery.

Oncocytoma

▶ Incidence. Oncocytoma is the second most common benign solid renal tumor, is mostly discovered incidentally, and represents approximately 3% to 7% of all renal tumors with a male predominance of about 2:1.

▶ Findings and differential diagnosis. Oncocytomas are extremely difficult to differentiate from other renal lesions, especially malignancies. Prolonged contrast enhancement is also seen in clear cell RCC, and rim enhancement occurs mainly in renal carcinoma but is also found in angiomyolipoma (Fig. 14.18) and occasionally in oncocytoma (Fig. 14.20). A centripetal enhancement can be seen in angiomyolipoma and RCC as well. A hyperenhancement can be observed in oncocytomas and RCC.⁵⁶

Tikkakoski et al found a central scar in only 1 of 36 renal oncocytomas, and a radial vascular pattern was noted in just 3 of the 36 (Fig. 14.21).¹⁰³ Because central necrosis and scars both represent devitalized areas, renal carcinomas with central necrosis may also look like oncocytomas. Thus, even small, central devitalized zones are not reliable indictors of oncocytoma.

Thus, differentiating oncocytoma from RCC and other solid renal neoplasms is not always possible with ultrasonography, CT, or MRI. The presence of a central scar and a spoke-wheel pattern of vessels on CEUS are often suggestive of oncocytoma but are not specific enough. A core biopsy may be helpful but is not generally recommended.

Renal Cell Carcinoma

▶ Incidence. RCC accounts for 2% to 3% of all malignant diseases in adults.⁵¹ From 1970 to 1990, its incidence in Europe rose from 6 to 7 per 100,000 to 8 to 9 per 100,000.³³ This is attributable more to a higher detection rate based on the widespread use of CT, MRI, and sonography than to an actual increase in disease. The percentage of incidentally detected renal carcinomas has risen accordingly and is estimated at more than 70%. Approximately 75% of incidentally detected renal carcinomas are limited to the kidney, and the 5-year survival rate for stage I lesions is 70% to 80%.¹⁰⁴

Males predominate by about a 2:1 ratio. Approximately 90% of all solid renal tumors are malignant.⁸⁰ RCC is bilateral in 1.5% to 4% of cases. Adrenal gland invasion was found in 4.3% of 695 nephrectomized patients.⁸² A second malignant tumor eventually develops in 27% of patients with RCC.⁷⁵ Synchronous adrenal involvement occurs in 1% to 7%.^{48 ,​ 89} Patients on long-term dialysis have a three to four times higher risk of developing RCC and therefore require regular follow-ups.

▶ Etiology. A genetic disposition for developing RCC is mainly seen in patients with von Hippel-Lindau disease (35%–40%), where involvement is bilateral in approximately 75% of cases and multifocal in 87% (Bourneville-Pringle disease).⁵³ The incidence of RCC is also increased in leiomyomatosis, hereditary papillary carcinoma, and the very rare Birt-Hogg-Dubé syndrome.

▶ Classification. RCCs are classified into four histologic categories:

•Clear cell RCC (70%–80%)

•Papillary RCC (10%–15%)

•Chromophobe RCC (5%)

•Bellini duct carcinoma of the renal collecting ducts (1%)

Tumor angiogenesis is greatest in clear cell RCC, less pronounced in papillary RCC, and comparable in chromophobe RCC, which has the best prognosis of the renal cell cancers. Bellini duct carcinoma has little vascularity and a very poor prognosis.

The Fuhrman classification is a widely used system in which clear cell RCC is graded from 1 to 4 based on the nuclear properties of the tumor cells.²⁴ The higher the grade, the greater the degree of angiogenesis.⁵

▶ Findings. RCCs can have a broad range of sonographic appearances. The smaller the tumors, the more likely they are to appear isoechoic or hyperechoic to surrounding healthy renal tissue.²⁵ Larger tumors have a more heterogeneous echo pattern. Typically, they have low echogenicity with more hypoechoic or cystic (necrotic) areas, and up to 20% of larger tumors contain calcifications. RCCs with devitalized, necrotic components are believed to have an unfavorable prognosis.¹¹

Vascularity is occasionally detectable with color duplex sonography. Three types of perfusions are distinguished after contrast administration:

•Type 1: Synchronous enhancement with central ischemia, rim enhancement, and delayed washout (Fig. 14.22)

•Type 2: Delayed contrast wash-in, scant perfusion, rapid washout (Fig. 14.23)

•Type 3: Delayed, then increased enhancement with slow washout (Fig. 14.24)

Patients with an end stage renal disease have a four- to fivefold increased risk of developing renal cancer in their native kidneys, and should therefore be screened regularly.^{120 – 123}

Diffusely infiltrating RCCs are rare and difficult to classify sonographically because they are conspicuous not by their echogenicity or echo pattern but by their expansion of the renal parenchyma. They are less vascularized than the surrounding parenchyma, so the differential diagnosis should include urothelial carcinoma (p. 441) infiltrating the renal parenchyma.

▶ Vascular invasion. The detection of vascular invasion by RCC has major clinical implications and is linked to a mostly poor prognosis. Besides nephrotic syndrome and other diseases associated with hypercoagulopathy, RCC is the most frequent cause of renal vein thrombosis. RCC is also the leading cause of isolated vena cava thrombosis. The likelihood of tumor invading the renal vein is 4% to 15%. The extension of tumor thrombosis into the inferior vena cava is seen in 7% of cases.⁴¹ Because of local anatomy, this occurs much more often on the right side than on the left (shorter distance, no “physiologic” constriction where the renal vein crosses the aorta). If the thrombus extends into the right atrium, operative treatment will require coordination with a cardiothoracic surgeon. Approximately 90% of tumor thrombi in the vena cava are infradiaphragmatic. If vena cava thrombosis occurs, this usually signifies that the renal cell tumor has attained considerable size (mean diameter 10 cm).⁸⁰

Frequently, renal vein thrombosis is already apparent in the B-mode image. The B-mode examination is followed by color duplex vascular imaging. Color duplex sonography has a reported sensitivity and specificity for renal vein thrombosis of 73% and 100% on the right side and 75% and 94% on the left side, respectively.²⁸ One limiting factor is that, while larger thrombi are clearly visualized on B-mode in duplex scans, smaller thrombi that are centrally located may be obscured due to blooming artifact. An ipsilateral varicocele may develop secondarily.⁷⁴ In patients with renal vein thrombosis, diastolic flow is missed and the RI number is 1.

Ultrasound contrast agents can demonstrate the vascularity of venous tumor invasion and pathologic enhancement of regional lymph nodes, analogous to observations in CT. CEUS is the modality of choice in terms of detecting residual flow as well as thrombus extension toward the vena cava (Fig. 14.25).

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